1. Field of the Invention
The present invention deals generally with heat transfer devices for transporting heat by a condensable working fluid, and more specifically with a heat transfer device, in which a liquid phase working fluid is refluxed mainly by gravity, to a heated portion where the heat is transferred from outside.
2. Discussion of the Related Art
As a heat transfer device, heat pipes are well known in the art, which transport heat in the form of latent heat of a working fluid. In the heat pipes, a non-condensable gas is evacuated from an airtight container, and a condensable fluid such as water or hydrocarbon is encapsulated therein. Therefore, if the heat is transported to a part of the heat pipe from outside while cooling another part, the working fluid is vaporized by the transported heat, and the vapor flows to a cooled part where a temperature and a pressure are low. The vapor releases the latent heat outside of the container and then liquefies. The resultant liquid phase working fluid flows back to a so-called “heated portion” where the heat is transmitted from outside.
As described above, the working fluid vapor is transmitted to a heat radiating side by a pressure difference in the container arising from the input and radiation of heat. Meanwhile, a pressure for refluxing the liquid phase working fluid to the heated portion is required so that common heat pipes are adapted to create a capillary pumping. Specifically, thin slits, porous materials or meshes are arranged in the container so as to function as a wick. If the working fluid infiltrating into the wick is evaporated, the meniscus of the working fluid infilling pores in the wick comes down. Consequently, a capillary pumping arises from a surface tension. The condensed working fluid infiltrating into the wick is aspirated to the heated portion side by the capillary pumping thus created at the heated portion, and then flown back to the heated portion where evaporation takes place.
Also, heat pipes, in which the working fluid is refluxed by gravity, are known in the art. The heat pipe of this kind is called a thermosiphon. A structure of the thermosiphon is similar to that of the aforementioned heat pipe but does not comprise the wick. The thermosiphon is used in a gravitational field. In the thermosiphon, a lower end thereof in the direction of gravitational force is the heated portion, and the heat is radiated outside from its upper end. In the thermosiphon, accordingly, a working fluid vaporized by the heat transmitted from outside flows to the upper end where the temperature and the pressure are low in consequence of the outgoing radiation. As a result, at the upper end of the thermosiphon, the heat of the vaporized working fluid is released and the working fluid is condensed. Then, the working fluid is dropped or flown down by gravity, to the heated portion of the lower end of the container. Additionally, the wick may be applied to the container of the thermosiphon in order to disperse the working fluid all over the heated portion.
As described above, in the heat pipe, the working fluid is circulated by a repetition of its evaporation and condensation, therefore, in principle, the heat is transported in the form of latent heat of the working fluid. In order to transport the heat continuously, therefore, ample amount of the working fluid has to be present in the heated portion. In other words, it is necessary to collect the working fluid at the lower end of the container in the thermosiphon. In case that a so-called “reservoir” is heated as the heated portion, pool boiling of the liquid phase working fluid occurs so that the working fluid vaporizes. The working fluid vapor flows upward from the reservoir portion. On the other hand, the working fluid liquefied at the upper part of the container drops or flows down toward the reservoir portion. Namely, the vapor and the working fluid flow in directions opposite to each other to form a counter flow. Thus, there are a number of factors that hinder the evaporation and the flow of the working fluid remains in the traditional thermosiphon. Therefore, there is a need for improvement to enhance the heat transporting capacity.
An art of improving the performance of the thermosiphon is disclosed in “International Journal of Heat and Mass Transfer 44 (2001) 4287–4311” by Kaviany et al. According to the disclosed thermosiphon, a porous layer having a periodically modulated thickness is applied as a wick, to an inner face of the lower part of the container i.e., the heated portion. Specifically, the porous layer is made from particles of several hundred micrometers consolidated by a sintering etc., and a base layer thereof is composed of one or two layers of the sintered particles. On the base layer, there are formed “stacks” (or cones) composed of over ten layers of sintered particles so that the thickness of the porous layer is periodically increases. The stacks are formed into pyramidal shape, which is tapered toward the top.
The container, which has the wick composed integrally of the base layer and the stacks at the bottom, is evacuated and then filled with a proper condensable fluid as the working fluid. Accordingly, the wick is impregnated entirely with the liquid phase working fluid by the capillary pumping. If the heat is transmitted to the bottom of the container under such condition, the heat is transmitted to the working fluid through the wick, and the working fluid is thereby heated and evaporated. The working fluid vapor flows toward the upper portion of the container and then contacts with the container so that the heat is drawn therefrom. Consequently, the working fluid is liquefies and it drops or flows down to the wick. The liquid phase working fluid coming down to the tip of the stack infiltrates into the stack, and forms a liquid film on the surface of the stack by the capillary pumping created at the surface of the stack.
Namely, droplets generated as a result of condensation of the working fluid fall onto the tip of the stack, while the liquid phase working fluid is pumped up by the capillary pumping from the base layer to the stack. Also, the heat transmitted to the bottom portion of the container is further transmitted to the stack from the base layer and the bottom side of the stack. Hence, the evaporation of the working fluid takes place principally at the portion of an outer circumferential face of the stack in the vicinity of a base portion. Consequently, the vapor flows upward through interspaces (i.e., valleys) between the individual stacks.
In other words, the working fluid is evaporated from the thin liquid film of the working fluid formed on the outer circumferential face of the base portion of the stack, and the liquid phase working fluid is supplied to the evaporating portion by the capillary pumping generated at the porous structured stack. Therefore, the liquid phase working fluid can be evaporated efficiently without a choking of the liquid flow. Moreover, the working fluid vapor ascends through the so-called “valley portion” between the stacks so that it rarely conflicts with the liquid phase working fluid flowing back to the wick. As a result, the circulation movement of the working fluid is smoothened so that the heat transporting characteristics is thereby improved.
In the aforementioned thermosiphon having the wick comprising stacks, the working fluid is evaporated principally at the lower part of the outer circumferential tapered face of the stack. Hence, there is a need for forming the thin liquid film of the working fluid stably on the tapered outer circumferential face of the stack. However, according to the prior art, the reflux of the working fluid to the stacks mainly depends on free-fall from a heat radiating portion (or a condensing portion) formed at the upper part of the container and the capillary pumping generated in the wick. For this reason, in case the thermosiphon is inclined, or in case a heat flux is large, a flow rate of the working fluid back to the stacks becomes insufficient and this shortage makes it difficult to form the liquid film of the working fluid on the outer circumferential face of the stack. As a result of this, a heat transporting performance may be degraded.